Generally, a sector servo method, or a servo method conforming thereto, for positioning a data head sector based on servo data is used in information storage devices such as a magnetic disk unit to achieve high density recording. The servo data is stored in advance at the end of the sector formed on a data recording plane. The servo method is suitable for a seek operation for rapidly moving the head to a target track, and to a following operation for causing the head to follow the center of the target track with high precision.
In the seek operation, the speed of the head is controlled in accordance with a predetermined target speed curve in order to move the head to the vicinity of the target track. When the head reaches the vicinity of the track, the operation is switched to the following operation in order to control the head's position and to cause it to follow the center of the track. A target speed signal is calculated from the target speed curve based on the difference (remaining distance) between a current position and a target position of the head. A speed error signal, which is a deviation between the target speed signal and an actual speed signal of the head, is amplified and is applied as a feedback control input to a voice coil motor for driving the head.
Also, when the target speed curve is given in a ramped manner during deceleration, i.e., when decelerated at a constant acceleration, a speed deviation is produced in the above-mentioned feedback control. Two methods are contemplated for reducing the speed deviation. One method widens a control band of the speed loop and the other method reduces an inclination of the target speed curve. The former has a problem in that there is a limit in the width of the control band due to a mechanical resonance of a support mechanism system for supporting and driving the head, and the latter has a problem in that it is difficult to keep a head access time within a predetermined time.
A proposed control method of finding a target acceleration curve for advance use in the speed control by differentiating the target speed curve and a method of obtaining an acceleration feed forward signal from the target acceleration curve in order to apply it to a speed error signal has been described in Japanese Patent Laid-Open No. 58-182169 for example. Its purpose is to compensate the stability through the use of feedback control and to improve the response through the use of feed forward control. The known method allows the head to follow the target speed curve accurately without needing to widen the control band of the speed control loop nor reduce the inclination of the target speed curve. Thus, the known method causes less speed error in switching to the positional control and prevents an overshoot phenomenon in which the head movement exceeds a target track position or an undershoot phenomenon in which the head stops before reaching the target track position, as well as reducing the access time.
Further, it is possible to accelerate/decelerate the head to cause it to follow the center of the target track only by using the positional control system without controlling the seek operation and the following operation by separate control systems. This method eliminates the switching of the control systems and allows the head to be settled on the center of the target track smoothly and allows the access time to be reduced. An example thereof is described by Hirata et. al in "Head Positioning Control of Hard Disk Using H.infin. Control Theory", Papers of the Society of Instrument and Control Engineers, Vol. 29, No. 1, pp. 71/77 (1993). In the paper, a compensation filter derived from the H.infin. control theory is used as a feedback control compensator to suppress resonance/disturbance of the head supporting mechanism system. A smooth sinusoidal acceleration signal is input in feed forward control to move the head at high speed.
It is known to design a control system based on dynamic characteristics of an object to be controlled so that a given design specification is met. For example, a speed control system of a magnetic disk unit is designed so that zero-cross frequency in an open loop characteristic meets with the design specification by deciding a speed gain of a speed loop by using a loop gain of an object to be controlled and by deciding a loop gain of an acceleration feed forward compensator by using an inverse gain of the loop gain of the object to be controlled. The loop gain of the object to be controlled here means a loop gain from a control input to a speed signal of the head. The loop gain is given by an amplifier gain of an amplifier, a force constant of a voice coil motor, an equivalent mass of the head and a speed detection gain, etc. As methods for detecting the speed, there is known a method of calculating a position signal by backward differential or a method of calculating it by using a speed observer from a position signal and a current signal from an amplifier.
The loop gain of the object to be controlled varies depending on manufacturing allowances, operating conditions, operating environments and an elapsed change, etc. For example, the force constant of the voice coil motor changes depending on the track position where the head is located due to magnetic flux leakage from both ends of the voice coil. That is, a force gain is small at the inner and outer diameter sides of the disk, and is large at the center of the disk. A difference of these gains amounts to about 10%.
A position detection gain of the head depends on the manufacturing allowance of the head and the gain changes due to a dispersion of a core width of the head. That is, when the core width of the head is larger than a designed standard, both the read voltage and position detection gain become larger. By contrast, when the core width of the head is smaller than the designed standard, both the read voltage and position detection gain become smaller. A difference of these gains amounts to about 20% at the data head. A fluctuation of the speed detection gain of the head depends on the fluctuation of a detected gain of the position signal of the head, and is about the same with the detected gain of the position signal.
Mechanical characteristics of the voice coil motor and the electrical characteristics of a driving circuit, as well as others, also fluctuate as they are influenced by environmental factors such as the temperature and humidity within a unit. Further, when such a magnetic disk unit is used for a long period of time, the characteristics of the bearing supporting the head gradually changes due to abrasion and change in the oil characteristics. As a result, the characteristics of an object to be controlled become different from those when the unit is first shipped out.
That is, the characteristics of the entire control system deviate from the optimum point due to the dispersion of mechanical characteristics of the driving system for driving the head, the dispersion of electrical characteristics of the driving circuit and the dispersion of the detected gain of the head. These deviations prevent the performance of the rapid seek operation and the high precision following operation, thus degrading the performance of the unit. The dispersion of the response of the head may be suppressed more or less by performing the seek operation by using only the feedback control because the gain fluctuation of the object to be controlled is suppressed by the feedback characteristic.
For the case when the head is rapidly sought by using the feed forward control as described in Japanese Patent Laid-Open No. 58-182169 and in Papers of the Society of Instrument and Control Engineers, Vol. 29, No. 1, pp. 71/77 (1993) mentioned above, the dispersion of the gain of the object to be controlled turns out directly as a dispersion of the response of the head.
Static and dynamic external forces are applied to the head during the seek operation. A signal line for transmitting recording/reproduction signals of the head to a circuit board is provided by a cable composed of a copper foil pasted to a soft medium called an FPC (flexible printed circuit). The cable acts as a spring force, i.e. a force disturbance, to the head supporting mechanism. Due to the external force, the head speed signal deviates from a target speed signal during the seek operation using speed control. On the other hand, the head position deviates from a target position during the seek operation using position control. While the head turns about an axis of rotation of the head supporting mechanism section, a moment force is generated at the supporting mechanism section during the seek operation. As a result, the base supporting the supporting mechanism section vibrates. Due to the vibration of the base, a disk mounted on the base vibrates, thus causing a relative position error between the disk and the head as a positional disturbance to the head position.
Accordingly, it is an object of the present invention to eliminate the above-mentioned problems and to significantly increase the recording density of the disk unit as well as to rapidly position the head with high precision. In practice, it is an object of the present invention to provide an adaptive control system and an information storage device equipped therewith which can automatically correct the deviation of the control characteristics from the optimum point due to the dispersion of parts composing the positioning mechanism system and the electrical system, while always having good head positioning characteristics.
It is another object of the present invention to provide an adaptive control system and an information storage device equipped therewith which controls an acceleration feed forward gain in real-time by an adaptive controller in order to reduce the dispersion of the response of the position during the seek operation in which speed control is applied.
It is still another object of the present invention to provide an adaptive control system and an information storage device equipped therewith which controls an acceleration feed forward gain in real-time by an adaptive controller in order to reduce the dispersion of the response of the position during the seek operation in which position control is applied.
It is a further object of the present invention to provide a real-time adaptive control system and an information storage device equipped therewith which requires no training signal when a gain fluctuation of an object to be controlled is estimated.
In order to achieve the aforementioned objects, a first aspect of the present invention provides:
1) in a positioning adaptive control method for positioning a head for reading/writing information either recorded on a disk or to be recorded on the disk to a predetermined position of a target track by using a seek operation for positioning the head to a track on the disk and a following operation for positioning the head while on the track, a control system related to the seek operation is controlled sequentially during the seek operation so that the head may be positioned to the target track rapidly with high precision during the following operation which follows the seek operation.
A second aspect of the present invention for achieving the aforementioned objects is that a feed forward control input is controlled sequentially in a positioning adaptive control method for moving the head to a target position by adding a feedback control input obtained based on a speed error between a target speed and an actual speed of the head and a feed forward control input obtained based on a target acceleration. The second aspect has options as shown in items 2) through 6) below. It is noted that the feedback control input and the feed forward control input are used with any of the options.
2) The feed forward control input is controlled sequentially so that the speed error is reduced during head movement.
3) The feed forward control input is controlled sequentially so that a square of the speed error is minimized when the speed of the head starts to decelerate.
4) The feed forward control input is controlled sequentially by multiplying the feed forward control input with a control gain .theta.(k) which is expressed by .theta.(k+1)=.theta.(k)+.eta..multidot.uv(k).multidot.sa(k), wherein the feedback control input is denoted as uv(k), the feed forward control input as sa(k) and a learning gain as .eta..
5) The feed forward control input is controlled sequentially so that a value obtained when square values of the speed error are added sequentially is minimized when the speed of the head starts to decelerate.
6) The feed forward control input is controlled sequentially by multiplying the feed forward control input with a control gain .theta.(k) which is expressed by .theta.(k+1)=.theta.(k)+.gamma.(k+1).multidot.uv(k).multidot.sa(k), wherein the feedback control input is denoted as uv(k) the feed forward control input as sa(k) and a learning gain as .gamma.(k).
A third aspect of the present invention for achieving the aforementioned objects provides a feed forward control input which is controlled sequentially in a positioning adaptive control method for moving a head to a target position by adding a feedback control input obtained based on a position error between a target position and an actual position of the head and the feed forward control input obtained based on a target acceleration. Similarly to the second aspect, there are options also for this aspect as shown in items 7) through 11) below. The feedback control input and the feed forward control input are used with any option.
7) The feed forward control input is controlled sequentially so that the position error is reduced during head movement.
8) The feed forward control input is controlled sequentially so that a square of the position error is minimized when the head starts to move.
9) The feed forward control input is controlled sequentially by multiplying the feed forward control input with a control gain .theta.(k) which is updated sequentially by .theta.(k+1)=.theta.(k)+.eta..multidot.uv(k).multidot.sa(k), wherein the feedback control input is denoted as uv(k), the feed forward control input as ua(k), a learning gain as .eta. and the feed forward control input before a gain is applied as sa(k).
10) The feed forward control input ua(k) is controlled sequentially so that a value obtained when square values of the position error are added sequentially is minimized when the head starts to move.
11) The feed forward control input ua(k) is controlled sequentially by multiplying the feed forward control input sa(k) with a control gain .theta.(k) which is updated sequentially by .theta.(k+1)=.theta.(k)+.gamma.(k+1).multidot.uv(k).multidot.sa(k), wherein the feedback control input is denoted as uv(k), the feed forward control input as ua (k), a learning gain as .gamma.(k) and the feed forward control input before the gain is applied as sa(k).
A fourth aspect of the present invention for achieving the aforementioned objects is that feed forward control inputs are controlled sequentially in a positioning adaptive control method for moving the head to a target position by adding a feedback control input obtained based on a speed error between a target speed and an actual speed of the head or a position error between a target position and an actual position of the head, a first feed forward control input obtained based on a target acceleration and a second feed forward control input obtained based on an acceleration disturbance signal. The fourth aspect also has options as shown in items 12) and 13) below, and the feedback control input, the first feed forward control input and the second feed forward control input are used in any of the options.
12) The first and second feed forward control inputs are controlled sequentially so that the speed error is reduced during head movement.
13) The first and second feed forward control inputs are controlled sequentially so that the position error is reduced during head movement.
A fifth aspect of the present invention for achieving the aforementioned objects is that a feed forward control input is controlled sequentially in a positioning adaptive control method for generating a control input for moving the head to a target position by adding a feedback control input obtained or a position error between a target position and a head position based on a speed error between a target speed and a head speed and the feed forward control input obtained based on a target acceleration, and has the options as shown in items 14) through 17) below. A variable gain is provided at an arbitrary place from the control input for moving the head to the target position to the speed signal of the head, and the feedback control input and the feed forward control input are used in any of the options.
14) The feed forward control input is controlled sequentially so that the speed error is reduced even if the variable gain is increased or reduced during head movement to immediately reduce the speed error which has been increased due to a gain change of the variable gain.
15) The feed forward control input is controlled sequentially so that the speed error is reduced even if the variable gain is increased or reduced in advance in order to immediately return the head speed signal which has been fluctuating due to the gain change of the variable gain to the state before the gain change of the variable gain.
16) The feed forward control input is controlled sequentially so that the position error is reduced even if the variable gain is increased or reduced during head movement in order to immediately reduce the position error which has been increased due to the gain change of the variable gain.
17) The feed forward control input is controlled sequentially so that the position error is reduced even if the variable gain is increased or reduced in advance in order to immediately return the head position signal which has been fluctuating due to the gain change of the variable gain to the state before the gain change of the variable gain.
It is noted that in items 4), 6), 9) and 11) described above, an initial value of the control gain .theta.(k) may be set at .theta.(0)=1.0 and, again in items 4), 6), 9) and 11), a value of a final control gain controlled during the previous head movement may be used as an initial value of the control gain .theta.(k).
Signals handled in each positioning adaptive control method described above may be either digital signals or analog signals.
In items 1), 2), 3), 5), 7), 8), 10) and 12) through 17) discussed above, the sequential control may be carried out in synchronism with time intervals for reading position data in a sector servo.
A sixth aspect of the present invention for achieving the aforementioned objects provides:
18) a positioning adaptive control device of an information storage device comprising a recording medium in which position data is recorded in advance; a head for reading the position data; a position signal computing element for generating a position signal from the position data reproduced by the head; a speed signal computing element for generating a speed signal of the head based on the position signal; a target acceleration generator for generating a target acceleration signal based on a remaining distance between a target position and a current position of the head; a target speed generator for generating a target speed signal based on the remaining distance; a speed error amplifier for generating a feedback control input signal by amplifying a speed error signal of a deviation between the target speed signal and the head speed signal; and a feed forward controller for generating a feed forward control input signal by multiplying the target acceleration signal with a controllable variable gain. The head is moved to the target position by a control input signal obtained by adding the feedback control input signal and the feed forward control input signal. The control device further comprises an adaptive controller for sequentially controlling a variable gain of the controllable target acceleration signal so that the speed error signal is minimized.
19) In item 18), the adaptive controller may control the variable gain of the controllable target acceleration signal sequentially based on a value in which the speed error signal, the target acceleration signal and a predetermined learning gain are multiplied so that a square of the speed error signal is minimized.
20) In item 18), the adaptive controller may control the variable gain of the controllable target acceleration signal sequentially based on the speed error signal, the target acceleration signal and a predetermined learning gain so that a value in which squares of the speed error signal are added sequentially is minimized.
A seventh aspect of the present invention for achieving the aforementioned objects provides:
21) a positioning adaptive control device of an information storage device comprising a recording medium in which position data is recorded in advance; a head for reading the position data; a position signal computing element for generating a position signal from the position data reproduced by the head; a target acceleration generator for generating a target acceleration signal; a position controller for generating a feedback control input signal by filtering a position error signal of a deviation between a target position signal and the position signal; a feed forward controller for generating a feed forward control input signal by multiplying the target acceleration signal with a controllable variable gain. The head is moved to the target position by a control input signal obtained by adding the feedback control input signal and the feed forward control input signal. The control device further comprises an adaptive controller for sequentially controlling a variable gain of the controllable target acceleration signal so that the position error signal is minimized.
22) In item 21), the adaptive controller may control the variable gain of the controllable target acceleration signal sequentially based on a value in which the position error signal, the target acceleration signal and a predetermined learning gain are multiplied so that a square of the position error signal is minimized.
23) In item 21), the adaptive controller may control the variable gain of the controllable target acceleration signal sequentially based on the position error signal, the target acceleration signal and a predetermined learning gain so that a sum of squares of the position error signal is minimized.
An eighth aspect of the present invention for achieving the aforementioned object provides:
24) a positioning adaptive control device of an information storage device comprising a recording medium in which position data is recorded in advance; a head for reading the position data; a position signal computing element for generating a position signal from the position data reproduced by the head; a speed signal computing element for generating a speed signal of the head based on the position signal; a target acceleration generator for generating a target acceleration signal based on a remaining distance between a target position and a current position of the head; a target speed generator for generating a target speed signal based on the remaining distance; a speed error amplifier for generating a feedback control input signal by amplifying a speed error signal for a deviation between the target speed signal and the head speed signal: and a feed forward controller for generating a first feed forward control input signal in which an acceleration disturbance signal is multiplied with a controllable first variable gain, a second feed forward control input signal in which the target acceleration signal is multiplied with a controllable second variable gain and a feed forward control input signal obtained by adding the first feed forward control input signal and the second feed forward control input signal. The head is moved to the target position by a control input signal obtained by adding the feedback control input signal and the feed forward control input signal. The control device further comprises an adaptive controller for sequentially controlling a first variable gain of the controllable acceleration disturbance signal and a second variable gain of the controllable target acceleration signal so that the speed error signal is minimized.
A ninth aspect of the present invention for achieving the aforementioned objects provides:
25) a positioning adaptive control device of an information storage device comprising a recording medium in which position data is recorded in advance; a head for reading the position data; a position signal computing element for generating a position signal from the position data reproduced by the head; a target acceleration generator for generating a target acceleration signal; a position controller for generating a feedback control input signal by filtering a position error signal for a deviation between a target position signal and the position signal; a feed forward controller for generating a first feed forward control input signal in which an acceleration disturbance signal is multiplied with a controllable first variable gain, a second feed forward control input signal in which the target acceleration signal is multiplied with a controllable second variable gain, and a feed forward control input signal obtained by adding the first feed forward control input signal and the second feed forward control input signal. The head is moved to the target position by a control input signal obtained by adding the feedback control input signal and the feed forward control input signal. The control device further comprises an adaptive controller for sequentially controlling a first variable gain of the controllable acceleration disturbance signal and a second variable gain of the controllable target acceleration signal that the position error signal is minimized.
26) In items 18), 21), 24) and 25) above, the sequential control is preferably carried out in synchronism with time intervals for reading the position data in the sector servo.
A tenth aspect of the present invention for achieving the aforementioned objects provides an information storage device comprising a recording medium in which position data is recorded, a head for reading the position data and a positioning adaptive control device for positioning the head to a track on the recording medium. The information storage device is equipped with the positioning adaptive control device comprising components in any one of the items 18) through 26) discussed above.
According to the present invention, the acceleration feed forward signal is controlled in real-time during deceleration so that the speed error to a target speed is reduced when the head, whose speed is controlled, is shifted from an acceleration state or a constant speed state to a deceleration state in the positioning control system in which the seek operation is controlled by the speed control and the following operation is controlled by the position control. Thereby, the speed error caused by a fluctuation of the gain of an object to be controlled and/or disturbance may be minimized. Further, when the control is switched from the speed control to the position control in the vicinity of a target track, dispersion of responses of the head position and head speed after the switch may be fully suppressed, thus giving a favorable settlement response and an access time may be shortened.
The positioning may always be completed rapidly and with high precision by performing the sequential control described above during the seek operation and by performing the seek operation and the following operation ensuing thereto regardless of the fluctuation of gain of the object to be controlled.
According to the present invention, in a positioning control system in which the seek operation and the following operation are controlled using only the position control, the acceleration feed forward signal is controlled in real-time so that a position error with respect to a target position is reduced from the start of the seek operation. Thereby, the position error caused by a fluctuation of a gain of an object to be controlled and/or a disturbance may be minimized. Then, a favorable settlement response is given when the head reaches the target track and the head access time may be shortened.
According to the present invention, the previously used training signal becomes unnecessary. Further, because the acceleration feed forward signal is controlled in real-time during the seek operation, the head can follow a target orbit with high precision. Accordingly, the target orbit which has been loosely set in the past may be set more sharply and the access time may be shortened.
The specific nature of the invention, as well as other objects, uses and advantages thereof, will be clearly apparent from the description and from the accompanying drawings.